Numerous types of electromagentic sensors repetitively scan through a scan pattern to repetitively sense or examine a particular electromagnetic phenomena. Usually, the electromagnetic sensing equipment and processing equipment are used to recognize, measure and transmit information regarding the phenomena being sensed. The repetitive scanning or sensing may be related to a repetitious time interval or repetitious movement of some portion of the electromagnetic sensor. However, the repetitive sensing may be related to some other repetitious quantity of a specific type of electromagnetic sensor.
Frequency scanning receivers repetitively scan portions of the electromagnetic frequency spectrum while searching for the presence of electromagnetic energy radiated from a radio or radar transmitter. Searching and tracking radar systems often repetitively scan a particular spatial area or volume to detect radar signals that are transmitted or reflected by the phenomena to be sensed. Also, certain types of direction finding sensors repetitively scan a spatial volume to detect electromagnetic energy and to determine the direction of arrival of such energy. The above sensors are examples of electromagnetic sensors that repetitively scan through a scan cycle to examine an electromagentic phenomena.
Energy which is sensed by such scanning electromagnetic sensors is frequently perturbed by noise or other electromagnetic signals. It is often difficult to distinguish between pertubations and the electromagnetic radiation that is desired to be sensed. For example, in the case of a direction finding electromagnetic sensor, received signal amplitude pertubations are often imposed upon the signal by movement of a source antenna. The signal provided to the sensor receiving equipment is usually modulated by both the source antenna movement and by the sensor movement of the scanning apparatus. Furthermore, the scanning modulation may be obscured by the original source modulation.
In a direction finder using a spinner antenna as the electromagnetic sensor, the spinner antenna receives side lobe signal energy. Thus, the spinner antenna is receiving electromagnetic radiation from an area defined by a main lobe and by side lobes of the antenna. If the direction finder is receiving energy from a searching or tracking radar system, the received signal is modulated by the movement of the transmitter antenna as the radar searches or tracks. Furthermore, it is often difficult to determine the center of the direction in which the spinner antenna is pointed if the spinner antenna beam width is large.
Several techniques have been developed to minimize the effects of the aforementioned pertubations which interfere with scanning electromagnetic sensors. Many systems employ cathode ray tubes as displays with medium to long persistance to provide a visual memory of previous scan patterns so that an operator can choose from among several scan patterns to determine which scan pattern is most acceptable. Also, scanning electromagnetic sensors using antennas often employ a technique for cancelling all signal energy received through antenna side lobes, thus removing this perturbing factor. Such latter systems are generally expensive to implement.
Thus, a need has arisen for an inexpensive system for reducing the negative effect of source modulation, sensor side lobe responses and other perturbing influences on signals detected by a scanning electromagnetic sensor. In particular, a need has arisen for processing equipment for use in scanning electromagnetic sensors to reduce the negative effects of the above described pertubations. The processing equipment should provide an estimate or best approximation of the electromagnetic phenomena to be sensed. The scan pattern estimate should be narrowed with respect to the sensed electromagnetic scan pattern and should reduce or substantially eliminate the side lobe responses of a scanning antenna.